Human cancer-prone genetic diseases are being studied in order to identify groups of people with an increased susceptibility to environmental carcinogenesis, and to attempt to correlate such sensitivity with clinical features and with defective DNA repair. Patients with xeroderma pigmentosum and ataxia telangiectasia, diseases with ultraviolet and X-ray sensitivity, respectively, and with familial malignant melanoma (dysplastic nevus syndrome), and neurofibromatosis, diseases with no documented environmental sensitivity, are being studied. Detailed examinations of the clinical features of affected individuals are being made. Cultures of skin and blood are being established and the effects on cell survival, DNA repair, and chromsome abnormalities after treatment with DNA damaging agents (ultraviolet, bleomycin, and psoralen plus long wavelength ultraviolet) are being examined.